The laying of asphalt paving material on road surfaces entails spreading paving material consisting of an aggregate filled bituminous mixture on a prepared roadbed. The paving material is spread while hot and is then compacted so that upon cooling a hardened pavement surface is formed. Conventional paving machines utilize a heavy assembly termed a “screed” that is drawn behind the paving machine. The screed includes a replaceable screed plate that is constructed of a suitable steel, to spread a smooth even layer of paving material on the prepared roadbed. The weight of the screed assembly aids to compress the paving material and perform initial compaction of the paving material layer. Screed assemblies can include vibratory mechanisms placed directly on the screed plate or separate vibratory tamper bars connected in tandem with the screed plate to aid in the initial compaction of the paving material.
To facilitate laying of the paving material, the screed is typically heated, to a temperature in the range of about 82° to 171° C. (180° to 340° F.). Heating the screed assists the paving material in flowing under the screed and reduces adhesion of the paving material to the screed. If the screed is not adequately heated, the bituminous mixture contacts the bottom of the screed and begins to harden, resulting in buildup of paving material and excessive drag.
Conventional screed assemblies were commonly heated by fossil fuel powered burners that heat the upper surface of the screed plate by the direct application of flame or hot exhaust gases. More recently, screed assemblies with electrically powered heating elements are being used, wherein the heating elements are usually bonded or tightly secured to an upper surface of the screed plate.
For example, as shown in U.S. Pat. No. 5,417,516, a heated screed assembly for use with a paving machine includes a screed with an elastomeric, electrically-powered heating element carried on the upper surface of the screed. To ensure that movement of the heating elements along a plane of the upper surface is substantially prevented during operation of the screed assembly, and to also ensure that the heating elements stay in intimate contact with the screed while being vibrated during operation, a layer of insulation is placed on top of the heating elements and a retaining plate assembly, which is a heavy steel grid member, is placed on top of the insulation to hold the heating elements and the insulation in place. The design requires loose components placed on top of one another to maintain full contact of the heating elements with the screed.
Screed assemblies by nature operate in an extremely abusive environment that may easily cause damage to or failure of key components. The heating elements of a screed assembly are among the key components that may require repair or replacement. However, the heavy nature of the equipment involved and the design of conventional screed assemblies requires that any such maintenance must normally be carried at a depot or shop location, for example, wherein the entire frame of the screed assembly must be disassembled in order to repair or replace the malfunctioning heating element. Furthermore, the use of multiple pieces to adequately hold down the heating elements in conventional screed assemblies adds to the time consuming and labor intensive process to repair or replace heating elements.
The present invention is directed to overcome one or more of the problems as set forth above.